1. Field
Embodiments of the present invention relate to a lithographic apparatus and a method to compensate for the effect of disturbances on the projection system of a lithographic apparatus.
2. Background
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The forces required for scanning movements of movable objects of the lithographic apparatus, such as a substrate support, cause reaction forces and torques in the lithographic apparatus. Such reaction forces and torques may lead to disturbances in the lithographic apparatus. In particular, the disturbance forces may be propagated to other parts the lithographic apparatus, for instance to the projection system. The resulting movements of the projection system may lead to imaging errors and/or a decrease in throughput of the lithographic apparatus.
Similar reaction forces and torques and consequent movements of parts of a lithographic apparatus may result from the actuation forces of other movable objects of a lithographic apparatus. Such other moveable objects are, for instance, patterning device supports, patterning device masking devices, or robots for handling substrates or patterning devices.
In a prior art lithographic apparatus it has been proposed to provide a so-called balance mass. Such balance mass is connected to the movable object and is configured to move in a contra-direction, i.e. a direction opposed to the actuation direction, so that the reaction forces are compensated. These balance masses are provided for the main direction of movement of the respective moveable object, for instance the substrate support.
Due to the higher demands on wafer throughput, the accelerations of scan set-points increase, hence introducing higher disturbance forces. At the same time the required accuracy of lithography scanners increase resulting in lower allowable disturbance levels.
As a result of the increasing demands on wafer throughput and accuracy, balance masses may be required in non-dominant directions, i.e. non-main directions of movement of the respective movable object, for instance for rotational directions of the substrate stage, to meet these demands. The provision of such balance masses may result in a more complex and expensive construction. Also for some embodiments the compensation caused by a balance mass may no longer suffice.
It is remarked that U.S. Pat. Nos. 5,876,012, 6,322,060, 6,864,962, 6,937,317, 7,063,192, 7,264,235, 7,275,627, the contents of which are herein incorporated by reference in their entirety disclose lithographic or scanning apparatus wherein a disturbance compensation is performed by controlling an active vibration suppression device. None of these publications provides a solution for the above-identified drawback of a prior art lithographic apparatus.